1. Field of the Invention
The present invention relates to an exhaust port timing control apparatus for a two-cycle engine and particularly to an exhaust port timing control apparatus for variably controlling an exhaust port timing by way of a control valve disposed in an exhaust passage of the two-cycle engine.
2. Prior Art
As well known, since an exhaust port of the two-cycle engine is opened or closed by the reciprocating motion of a piston, the exhaust port timing is always constant with respect to the crank angle. Therefore, if the exhaust timing is designed so as to obtain a high power at the high speed operation, the fresh charge exits directly out of the exhaust port at the low speed operation, because the opening period of the exhaust port is rather long at low speed. This phenomenon is a so-called "short-circuiting". On the other hand, if the exhaust port timing is designed so as to obtain a smooth operation at low speed, the engine loses power at the high speed operation because of lowered exhaust efficiency.
Due to this inherent nature of the two-cycle engine, the two-cycle engine has been generally considered to be difficult to obtain a steady performance under any operating conditions.
However, in recent years there are several techniques in two-cycle engines that a steady performance can be secured through the whole operating conditions by means of positively controlling the exhaust port timing. There are several types of the exhaust port timing control apparatus such as a slide valve type, a rotary valve type, a flap valve type and the like.
The slide valve type is composed of a slidable valve coming near to or going away from the exhaust port and these techniques are disclosed in Japanese Examined Utility Model Applications, Jitsu-Ko-Sho 60-3314, Jitsu-Ko-Hei 4-20979, Jitsu-Ko-Hei 5-9458 and an Unexamined Patent Application Toku-Kai-Hei 1-159414.
The rotary valve type is composed of a valve body shaped in hourglass or cylinder which is provided in the exhaust passage and the exhaust port timing is controlled by the rotational angle of the valve body. This technique is disclosed in Japanese Unexamined Utility Model Application Jitsu-Kai-Sho 56-54336 and others.
The flap valve type is composed of a plate shaped valve body pivotally hinged at the rear end thereof and provided above the exhaust port. The exhaust port timing is controlled by the rotational angle of the flap valve around the pivot. This technique is disclosed in Japanese Examined Patent Application Toku-ko-Hei 5-61452 and in U.S. patent application Ser. No. 4,391,234.
However, those apparatuses have problems and disadvantages described as follows.
In the apparatus according to the slide valve, since the valve body is projected in the diagonal direction or in the horizontal direction to close the upper portion of the exhaust port, only step-by-step porting controls such as "low speed"--"high speed" or "low speed"--"medium speed"--"high speed" are available and a continuous control according to the engine speed can not be performed.
On the other hand, in case of the rotary valve type or the flap valve type, it is possible to control the exhaust port timing continuously, but it is impossible to obtain an accurate exhaust port timing, because these valve types have such a construction that the valve body is rotated or swinged in the upward or downward directions and therefore the clearance between the control surface and the cylinder wall surface is increased according to an increase of the rotation or swing angles. This will be described below using FIG. 8 and FIG. 9.
The flap valve 1 is disposed above the exhaust passage 2 and is swingable about the supporting pin 3. The control surface 1a is formed in a curvature along the inner circumference of the cylinder wall 4. The side edge portion of the control surface 1a is close to the side edge 2b of the exhaust port 2a. Referring to FIG. 9, for example, when the flap valve 1 swings downwards as much as 10 mm about the supporting pin 3 from the contact point with the cylinder wall surface, in accordance with the geometrical calculation, the clearance between the contact point and the cylinder wall surface, namely the exhaust port edge is 0.87 mm and the clearance between the side edge and the exhaust port edge is 0.95 mm. Thus, in case of the flap valve of one pin type, the clearances produced between the valve body and the cylinder wall surface can not be avoided. Therefore, it can not be avoided that the fresh charge leaks out of the cylinder through these clearances.